The present invention relates to a sample head for NMR tomography having a cage resonator comprising a number of conductor sections arranged on a cylinder surface and directed in parallel to the cylinder axis, electrically conductive screening means enclosing the conductor sections and arranged concentrically relative to the cylinder surface, and capacitors connected in series to the conductor sections which, together with the conductor sections, define the electric length of the cage resonator in such a manner that the cage resonator is resonant at a first, predetermined operating frequency f.sub.0, and having further means for exciting a TEM dipole wave in the cage resonator.
Different designs of such sample heads have been previously known, for example from DE- A 35 22 401 and EP-A 141 383. In the first case, the conductor sections extend over the full length of the cage resonator, and the capacitors are arranged between the ends of the conductor sections and the electrically conductive screening. In the latter case, a plurality of conductor sections is arranged axially behind each other and interconnected by the capacitors. In addition, the ends of the conductor sections are all interconnected by an annular conductor section.
In order to achieve a good signal-to-noise ratio, it is desirable that these cage resonators should have the highest possible quality (Q) and, accordingly, pronounced resonance. Consequently, they must be tuned to the operating frequency f.sub.0. In the case of the sample head known from DE-A 35 22 401, such tuning is effected by means of a tubular, electrically conductive section which is inserted into the one end of the cylinder formed by the conductor sections. This permits a tuning adjustment of the resonance frequency by a maximum of 3%. In the case of the sample head known from EP-A 141 383, one of the capacitors connected in series to the conductor sections is designed as a tunable capacitor permitting sharp tuning. However, such tuning must be effected in a separate test circuit, before installation of the test head.
The required sharp tuning of the sample head to the operating frequency has the result that each sample head is adjusted only to one such operating frequency, the latter resulting from the resonance frequency of the nuclear species to be observed, which resonance frequency is a function of the nuclear species as such and the magnetic field prevailing at the measuring point. In NMR tomography, one usually observes proton spins, and the magnetic field is selected in such a manner that the resonance frequency of the proton spins is either 100 MHz or 200 MHz.
However, in addition to observations of the density of proton spins, which is characteristic in particular for the distribution of water in examinations of biological tissue, observations of spins of other nuclear species, such as deuterium, carbon 13, phosphorus, fluorine, etc., are also of interest in NMR tomography. A field of particular interest is the observation of fluorine resonance which has a frequency lower by approximately 6% than the resonance frequency of protons. In practice, fluorine resonance observations are connected with considerable difficulties because the sample head, which is normally tuned to the resonance of proton, is not suited, for the reasons outlined above, for observations of the resonance of fluorine so that it has to be exchanged against a sample head especially tuned to the resonance of fluorine. However, the labor input required in this connection is so important that it practically prohibits the frequent exchange of the sample heads which would be required for carrying out successive proton and fluorine measurements on the same object.